Wind power plant having a plurality of wind turbine generators and a power plant controller

ABSTRACT

A wind power plant supplies power to a utility grid in accordance with an active power reference, where the wind power plant comprises a plurality of wind turbine generators and a power plant controller arranged to send an active power set point to the wind turbine controller of each of the plurality of wind turbine generators.

FIELD OF THE INVENTION

The present invention relates to a wind power plant having a pluralitywind turbine generators and a power plant controller with a dispatcher.

BACKGROUND

In an electrical utility grid, consumers can usually consume electricpower in an uncontrolled manner. Since hardly any energy is stored inthe grid, there can be no imbalance between the power produced and thepower consumed.

Therefore, the momentary production of power shall match the momentarypower consumption. Overproduction leads to an increase of the gridfrequency beyond the nominal value (e.g. 50 or 60 Hz), since theconventional synchronous generators accelerate, while over consumptionwill lead to a decrease of the grid frequency beyond the nominal value(e.g. 50 or 60 Hz), since the conventional synchronous generators willthen decelerate.

In order to stabilize the frequency of the electrical grid,conventionally about 10% of the producers contribute to what is called“primary power control”. These producers, also referred to as “primarycontrollers”, increase power output when the frequency falls below thenominal value and decrease power output when it rises above the nominalvalue.

Wind turbine generators (WTG) can be used to provide energy to theelectrical utility grid. Wind turbine generators are sometimes referredto as wind turbines (WT). A plurality of WTGs can form a wind powerplant (WPP) also known as a wind park or a wind farm.

A Power Plant Controller (PPC), which operates as a wind power plant(WPP) controller, generates proper active power set-points to all WTGs,in order to allow the plant active power follow certain active powerreference on the plant level. The part of the PPC which generates theactive power set-points to all WTGs is called a dispatcher. Thedispatcher coordinates all kinds of control modes, for instance,frequency control mode, active power curtailment mode, active power fastde-rating control mode, and so on.

It is thus highly relevant to provide methods to control a wind powerplant in a manner which ensures improved control modes by selecting anddispatching proper active power reference to a plurality of wind turbinegenerators in a WPP, so the aggregated active power of the power plantmatch the active power reference. Furthermore, it is relevant to providea wind power plant which reacts fast to changes in the requested activepower.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription.

The dispatching strategy described in this invention has the advantagethat the plant level active power follows the plant active powerreference with high accuracy. This is done in order to satisfytransmission system operator's (TSO's) Grid Integration Requirements(GIR). Fulfilling GIRs can be difficult as wind varies for eachindividual WTG and thus individually influence each WTG's power outputdifferently.

A first aspect relates to method for operating a wind power plant whichsupplies power to a utility grid in accordance with an active powerreference, the wind power plant comprises a plurality of wind turbinegenerators and a power plant controller arranged to send an active powerset point to the wind turbine controller of each of the plurality ofwind turbine generators, the method comprises the steps of:

-   -   operating the wind power plant with an active power reference        lower than a nominal active power of the wind power plant by        curtailing at least some of the wind turbine generators of the        plurality of wind turbine generators,    -   determine a power capability of each of the curtailed wind        turbine generators,    -   defining a first group comprising wind turbine generators having        a power capability higher than a power capability limit,    -   defining a second group comprising wind turbine generators        having a power capability lower than a power capability limit,    -   equalize curtailment of the curtailed wind turbines generators        by decreasing the active power set-point for the wind turbines        generators in the first group and increasing the active power        set-point for the wind turbines generators in the second group.

An embodiment of the first aspect determines a power capability of eachof the curtailed wind turbine generators comprise

-   -   calculating a positive power capability, as the difference        between possible power and production power and/or    -   calculating a negative power capability, as the difference        between minimum power and production power.

Possible power is to be understood as the power which a WTG are able todeliver if a high enough set point were to be forwarded to it.Production power is to be understood as the power which the WTG producesat the moment. Minimum power is to be understood as the minimum possiblepower which the WTG can produce under normal operation conditions.

An embodiment of the first aspect is setting a constant, prior the stepof equalize curtailment of the curtailed wind turbines generators, toensure that only a portion of the power capability, defined by theconstant times the power capability, is equalized in a dispatcheriteration.

An embodiment of the first aspect is limiting the decrease or theincrease in the active power set-point by an equalizing limit value in adispatcher iteration.

An embodiment wherein the step of limiting the decrease or the increasein the active power set-point is set as a minimum of the equalizinglimit value and the portion of the power capability.

A second aspect relates to a wind power plant having a plurality of windturbine generators and a power plant controller arranged to communicatewith the plurality of wind turbines generators, wherein

-   -   each wind turbine generator in the plurality of wind turbine        generators being related to a wind turbine controller, the wind        turbine controller being arranged to control an active power        output in its related wind turbine generator according to an        active power set point received from the wind power plant        controller;    -   the wind power plant supplies power to a utility grid in        accordance with an active power reference;    -   the wind power plant having an active power reference lower than        a nominal active power of the wind power plant, such that at        least some of the wind turbine generators of the plurality of        wind turbine generators are curtailed,    -   each of the curtailed wind turbine generators have a power        capability,    -   the curtailed wind turbine generators are comprised in,        -   a first group comprising wind turbine generators with a            power capability higher than a power capability limit, or        -   a second group comprising wind turbine generators having a            power capability lower than a power capability limit, and    -   the wind turbine controller being arranged to equalize        curtailment of the curtailed wind turbines generators by        decreasing the active power set point for the wind turbines        generators in the first group and increasing the active power        set point for the wind turbines generators in the second group.

The individual aspects of the present invention may each be combinedwith any of the other aspects. These and other aspects of the inventionwill be apparent from the following description with reference to thedescribed embodiments.

Any of the attendant features will be more readily appreciated as thesame become better understood by reference to the following detaileddescription considered in connection with the accompanying drawings. Thepreferred features may be combined as appropriate, as would be apparentto a skilled person, and may be combined with any of the aspects of theinvention.

BRIEF DESCRIPTION OF THE FIGURES

The wind power plant and its method according to the invention will nowbe described in more detail with regard to the accompanying figures. Thefigures show one way of implementing the present invention and is not tobe construed as being limiting to other possible embodiments fallingwithin the scope of the attached claim set.

FIG. 1 shows a wind turbine generator according to the presentinvention.

FIG. 2 shows an embodiment of a dispatcher.

FIG. 3 shows curve with equalized curtailment.

FIG. 4 shows sorting of the WTGs.

FIG. 5 shows reference change sharing priority.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention pertain to a wind power plantwith a plurality of wind turbine generators (e.g. a variable-speed windturbine generator). The plurality of wind turbine generators (WTG) mayform a wind power plant (WPP). The wind power plant seeks to produce anaggregated power from all the wind turbine generators, and to ensurethat the measured power (P_measurement) at the Point of Common Coupling(PCC) is equal to the desired power (P_reference) at the PCC. This isdone by dispatching individual or common power set-points to each of theWTG. In other words, the PPC receives a request for a specific outputpower for the WPP and by measuring the power at the PCC it can determineif it fulfills the request. If not then the PPC change the power setpoint values for the WTGs.

The way the Power Plant Controller (PPC) generates the active powerset-points for the WTGs is one of the most essential control functionsinside the PPC. As shown in FIG. 2, using the dispatching strategy, thePPC active power dispatcher splits the plant level active powerreference into different active power set-points for each WTG(Psetpoint_WTG1 . . . WTGn). These set-points may be different for eachWTG in the WPP or identical for a two or more of the WTGs in the WPP.

From the whole wind power plant point of view, WTGs are utilized asactuators for active power (P) control. The WTGs' set-points generatedby an active power dispatcher are approached by WTGs with certain ramprate, when the wind conditions allow. However, if the wind conditionsprevent the WTGs to produce the requested active power production, thenthe dispatching strategy needs to be robust enough to adjust the activepower set-points. Therefore each WTG's different wind conditions mayalso be utilized by the dispatching strategy. By doing this, the WTGswith high available power are allowed to produce more power tocompensate the WTGs with low available power. The coordination betweenthe PPC dispatcher and the WPP power system, using WTGs as actuators, isshown in FIG. 2.

An objective of the dispatcher is to ensure that the demanded power(e.g. from the Transmission System Operator (TSO)) is delivered as fastas possible, this applies both to increase and decrease in the activepower reference. Thus the objective of the dispatcher can be seen as amethod for controlling power ramp rates.

An issue of the prior art is the design principle for equalizing WTG setpoints sample by sample. The prior art equalizing set-points principlehas the following problems:

-   -   a) Equalizing WTG set-points causes low wind turbines to be        running at possible power where as high wind turbines will be        curtailed. This will cause losses as well as reduce the wind        farm ability to respond to step change where production need to        increase as turbines already producing at possible power will        not be able to perform.    -   b) When a wind farm is curtailed and one or more WTGs come out        of de-rate (e.g. due to single WTG de-rate or feeder limits) or        are released from pause mode, the rest of the WTGs will reduce        power to maintain the active power set point of the power plant.        Because these WTGs usually reach their set point before the        WTG(s) coming out of curtailment, a dip in power can occur.    -   c) When starting up the WPP and when turbines are changing from        pause to run mode, after a few turbines are running, a set point        that is too high will be calculated for the consecutive turbines        starting to produce. At this time the production running        turbines will need to be decreased while new turbines are        ramping up to meet the average set point. As running WTGs will        likely have further to ramp down compared to the starting WTGs        ramping up, an overshoot can occur.

Where in one aspect the dispatcher communicates active power set-pointsto the WTGs based on a set of weighting factors.

The dispatcher uses in some embodiments the surplus of available powerfrom a first subset of wind turbine generators inside a wind power plantto compensate for the lack of power in a second subset of wind turbinegenerators, thus regulating the WPP power output to a desired value.

In other words, the first subset of wind turbine generators can becontrolled in order to compensate the fluctuations in the second subsetof wind turbine generators, with the overall objective of ensuring thedesired value of total power output from the wind power plant.

FIG. 1 shows, an exemplary variable-speed wind turbine generator (WTG) 1is one of a plurality of wind turbine generators of a wind power plant(WPP) 2. It has a rotor 3 with a hub to which, e.g., three blades 4 aremounted. The pitch angle of the rotor blades 4 is variable by means ofpitch actuators. The rotor 3 is supported by a nacelle 5 and drives agenerator 12 via a main shaft 8, a gearbox 10, and a high speed shaft11. This structure is exemplary; other embodiments, for example, use adirect-drive generator.

The generator 12 (e.g. Induction or synchronous generator) produceselectrical output power of a frequency related to the rotation speed ofthe rotor 3, which is converted to grid frequency (e.g. about 50 or 60Hz) by a converter 19. The voltage of the electric power thus producedis up-transformed by a transformer 9. The output of the transformer 9 isthe wind turbine generator's terminals 9 a. The electric power from thewind turbine generator 1 and from the other wind turbine generators (WT2. . . WTn) of the wind power plant 2 is fed into a wind power plant grid18 (symbolized by “a” in FIG. 1). The wind power plant grid 18 isconnected at a point of common coupling 21 and an optional further stepup transformer 22 to a wind power plant external electrical utility grid20. The grid 20 can be equipped with regulation capacity againstgrid-frequency fluctuations, e.g. in the form of conventional producerswhich can increase and lower production on a short-time scale to controlfrequency.

A control system includes a wind turbine controller 13 and a wind powerplant controller (PPC) 23. The wind turbine controller 13 controlsoperation of the individual wind turbine generator 1, e.g. selectsfull-load or partial-load operation modes, depending i.a. on the currentwind speed. In the partial load mode, operation of the wind turbinegenerator at the optimal working point by adjusting the blade angle andcontrolling the tip speed ration to the aerodynamic optimum at thecurrent wind speed, and controls the converter 19 to produce electricityaccording to prescriptions of the power plant controller 23, e.g. aninstruction to provide a certain amount of active power and/or a certainamount of reactive power in addition the active power, etc. The windturbine controller 13 uses different input signals to perform itscontrol tasks, for example signals representing current wind conditions(e.g. from an anemometer 14 and a wind vane 15), feed-back signalsrepresenting pitch angle, rotor position, amplitudes and phases of thevoltage and current at the generator 12 and the terminals 9 a, etc., andcommand signals from the wind power plant controller 23. The power plantcontroller 23 receives signals representative of the voltage, currentand frequency at the point of common coupling 21 (parameters which maybe considered to represent the voltage, current and frequency in theutility grid 20) and receives information, requirements or commandsignals from a TSO or another operator (at “c” in FIG. 1). Based on someof these and, optionally, further input parameters the power plantcontroller 23 monitors the grid 20 and, upon detection of anon-compliance with the commands from a TSO or non-compliance with anyother grid requirement, the PPC 23 may command the wind turbinecontrollers 13 of the wind turbine generator 1 and the other windturbine generators of the wind power plant 2 (at “b” in FIG. 1) tochange operation of the output power supplied. Upon receipt of such acommand the wind turbine controller 13, may adjust the power outpute.g., by adjusting the blade-pitch angle, to comply with the wind-parkcontroller's command. Thus, in the exemplary embodiment of FIG. 1 thecontrol task of the control system to ensure that the TSOs GridIntegration Requirements are complied with. In other embodiments thiscontrol task is performed by the wind turbine controller 13 alone; inthose embodiments, the “control system” is represented just by the windturbine controller 13, without a power plant controller 23. In theseembodiments the wind turbine controllers 13, so to speak, take over thefunctionality of the PPC 23.

Although the wind turbine generator 1 shown in FIG. 1 is expected tohave three blades 4, it should be noted that a wind turbine generatormay have different number of blades. It is common to find wind turbinegenerators having two to four blades. The wind turbine generator 1 shownin FIG. 1 is a Horizontal Axis Wind Turbine (HAWT) as the rotor 4rotates about a horizontal axis. It should be noted that the rotor 4 mayrotate about a vertical axis. Such a wind turbine generators having itsrotor rotate about the vertical axis is known as a Vertical Axis WindTurbine (VAWT). The embodiments described henceforth are not limited toHAWT having 3 blades. They may be implemented in both HAWT and VAWT, andhaving any number of blades 4 in the rotor 4.

FIG. 2 exemplifies the arrangement of the calculation modules inside adispatcher. It can be seen that the dispatcher is disclosed as beingpart of a PPC. The dispatcher receives a plant level P reference that inthe Active power dispatcher module calculate the active power set pointPsetpoint WTG1 . . . WTGn, based on a plurality of status feedbacksignals Pavailable_WTG1 . . . WTGn. Where Pavailable_WTG is the activepower available from a specific WTG at the given time, calculated basedon the current wind speed and other parameters limiting the WTGproduction, and the available active power of the power system istherefore the aggregated Pavailable_WTG1 . . . WTGn for all the WTGs.The actual production (production power) of the WTGs Pproduction_WTG1 .. . WTGn is fed to a feeder line, which is connected to a Point ofcommon coupling (PCC) via a transformer. At the PCC the aggregatedactive power production is measured by means of a power meter. Themeasured active power (P measurement) is communicated to the PPC.Throughout this description, active power reference is used for thedemanded power for the wind power plant, whereas active power set-pointis used for the demanded power for the individual WTG.

The wind turbine generator (e.g. a variable-speed wind turbinegenerator) which supplies power to an electric grid which may beequipped with regulation capacity against grid-frequency and activepower fluctuations. “Electric grid” or “grid” is a utility grid outsidethe boundary and point of common coupling of a wind power plant; whenreference is made to the grid within a wind power plant an expressionwith explicit indication to the wind power plant is made, e.g.,“wind-park grid”.

As the present text deals with active power rather than reactive power,active power is briefly referred to as “power”, or “output power”. Wherereactive power is addressed, it is explicitly referred to as “reactivepower”. Although the claims refers to active power, implicitly this alsomeans that a change in a reactive power reference, voltage or frequency(or other changes) may, depending on the circumstances, result in achange in the active power reference, as these measures are linked.

There is an upper limit to the output power which can be produced by thewind turbine generator according to the embodiments, e.g. due tostructural limits and a current limit in the wind turbine generator'selectric converter. This amount of power is referred to as “nominalpower”. The wind speed sufficient for the wind turbine generator toproduce the nominal power is referred to as “nominal wind speed”. Whenthe wind turbine generator according to the embodiments operates at windspeeds above the nominal wind speed, only a fraction of the availablewind power is transformed to electric output power which corresponds tothe nominal power. This reduction of power production is, e.g., achievedby gradually changing the rotor-pitch angle towards the so-called flagposition. In other words, the wind turbine generator intentionally isnot operated at optimum efficiency. In some embodiments the wind turbinegenerator is also operated at a sub-optimal tip-speed ratio so as toreduce structural loads.

By contrast, during operation in partial load, at a wind speed below thenominal wind speed, the wind turbine generator according to theembodiments is operated at optimum efficiency. Meaning that it isoperated with the aerodynamically optimal blade pitch angle andtip-speed ratio.

In relation to control of the power plant (performed by the PPC) thereare several factors which can have negative impact on the PPC's activepower dispatching, and thus prevent an accurate control of the WPP. Forexample, each WTG may not necessarily be able to follow the active powerset-point even if the wind speed is high enough for the requiredproduction. The can be due to specific conditions in the turbine suchas: A comparability problem between the communication delays on theWTGs' feedback signals and the PPC active power control loop time, aninaccuracy of the WTGs' feedback signals generated from WTGs,uncertainty in the starting up duration for the WTG. Due to these issuesthe PPC's active power control encounters significant challenges.Therefore, the active power dispatching strategy has been focused anddeveloped with these issues in mind.

In an embodiment wherein the P_reference changes, e.g. that the WPPneeds to deliver more or less power. The PPC then needs to dispatch newpower set-points to the WTGs in order to comply with the demanded power.

In case where the P_reference is increased it is based on the aggregatedP_available such that the WTG contribute to the increased power by aweighting factor which depends on a relationship between the WTGsavailable power and the available power of the entire plant.

In case where the P_reference is decreased it is based on the P_producedsuch that WTG decrease its contribution to the WPP output by a weightingfactor which depends on a relationship between the WTGs produced powerand the power produced of the entire plant. E.g. depends on the fractionP_produced for a WTG divided by the power produced by the WPP.

These embodiments differs in that the turbines in the embodimentscontribute to the increase or decrease in power based on their totalavailable power such that the WTGs that are able to contribute the mostdo so.

In an embodiment the weighting factor is identical for a plurality ofthe WTGs. In a further embodiment, the weighting factors are differentfor each wind turbine generator.

In an embodiment, the weighting factor further depends on an estimatedwear of each wind turbine.

The active power reference change may be shared by the weightingfactors, according to each WTGs available wind power size or powerproduction size contributing to the whole plant available wind power orthe whole site power production. In other words, if a WTG's P_available(or P_production) contributes 10% of the plant level total availablewind power (or total P_production), then WTG′ may be sharing 10% of theactive power reference change.

FIG. 5 illustrates how a change in active power reference (ΔPref orΔP_reference) is shared among the WTGs, the percentage of each WTG'sPava (P_availble) or Pprod (P_production) contributing to the totalplant Pava (P total P_available) or total Pprod (total P_production) isused as a factor to calculate the WTG's set point. The example of a WTGwhich contributes 10% is seen for the WTG in the right side of FIG. 5.

It is also possible to use an algorithm in the strategy where weightingfactors are not used. It means all WTGs no matter their Pava or Pprodare high or low, they are treated relatively equally, there can bemainly two ways of the equalization:

First way, all the WTGs initially get equal set-point. However, the WTGswith higher Pava or Pprod have to compensate the WTGs with lower Pava orPprod. Thus the initial equalized set-point will be adjusted bycomparing with their Pava or Pprod, this adjustment can have severalrounds of iterations until either Pref is reached, or all WTGs reachtheir Pava limit.

Second way, all the WTGs equalize the curtailment in kW or MW orpercentage or p.u., so that the WTGs production over time will leveltowards the same average reduction from available/possible power toactual power production. As shown in FIG. 3, PcurtailWTGn att1=PcurtailWTGm at t1, and similarly PcurtailWTGn at t2=PcurtailWTGm att2.

This method will reduce the set-point of the high producing WTGs whileincreasing set-points for the low producing turbines with highproduction capability so that turbines equally share the productionreduction.

When to use the different modes such as weighting factor dispatcher orthe equalized curtailment dispatcher can be defined in the strategy, andit is possible to shift among the different curtailment strategies. Asan example, in the P control loop the active power reference (Pref) andthe active power measurement Pmeas are compared by establishing thedifference, this difference is termed as Perror (i.e.Perror=Pref−Pmeas). If the Perror is big, then the weighting factordispatcher can be used, if Perror is small, then the equalizedcurtailment can be used. Besides the Perror, other indexes can also beused to decide when to use the weighting factor dispatcher or theequalized curtailment dispatcher. It is also possible to use more thanone index to decide when to shift the curtailment modes.

In an embodiment the different modes: weighting curtailment and theequalized curtailment are both used and an index decides when to changemode.

In an embodiment the index is the power error, Perror, often the Perroris used the absolute value of the difference, i.e. without the sign.

Curtailment is defined by the active power set point being below nominalpower.

Two subsets are used for handling turbines during steady state control,a control subset and non-control subset. Turbines in the control subsetare turbines that are controllable and have control capability whereasturbines in the non-control subset are turbines having poor or nocontrollability. During steady state control all WTGs have theirrespective capability constantly evaluated to update the control andnon-control subsets. A WTG is moved to the non-control subset undereither of the following conditions:

-   -   1. It is producing less than Pmin    -   2. Its positive capability is less than a predetermined value

WTGs in the non-control subset are given a set-point of Pmin if theirproduction is less than minimum power, Pmin and given Possiblepower/Available power (plus PercentageOverNominalPower) otherwise. Theyare moved back to the control subset if their positive capabilityincreases above the controllable level. The predetermined value is a WTGspecific value, which may reflect the capability of the turbine; forexample, the turbine can have a low ramp rate when operating below aspecific power output meaning that it will not be suitable forregulating to power reference. The predetermined value may also bechosen in order to regulate the number of turbines in thenon-controllable subset.

In an embodiment a third non-communication subset is also used in allstates to handle turbines which cease communication.

In Steady-State Control the dispatching strategy focuses on controllingthe turbines to optimize the dynamic capability and is the strategy usedduring steady state control of the wind farm. In an embodiment thestrategy is to equalize the WTG production.

In an embodiment a distribute demand function distributes the activepower demand (as Pref−Pmeasured) from the control loop to the WTGs basedon their capability. When dispatching positive demand, i.e. increasingwind farm power, the turbine with the highest positive capability(Ppossible−Pproduction) is increased first with a weighted set point.The following next will be increased with a weighted lower set point andso on. This way the demand will be distributed to each WTG based on itscapability.

For dispatching negative demand, the turbine set point will be adjustedaccording to their negative capability (measurement-minimum power).

For a positive demand where the wind farm production must increase thedemand is distributed as:ActivePowerSP_(WTG)[n]:=PProduction_(WTG)[n]+demand*CapabilityPos_(WTG)[n]/TotalCapabilityPos

where n is the index of the WTGs

For negative demand where the wind farm production must decrease thedemand is distributed asActivePowerSP_(WTG)[n]:=PProduction_(WTG)[n]+demand*CapabilityNeg_(WTG)[n]/TotalCapabilityNeg

In an embodiment the power output from the WPP is generated in such away that each WTG or each WTG in the control subset is curtailed basedon a set power curtailment or a set fraction of its available activepower. Thus, the curtailed power for each of the WTG or each WTG in thecontrol subset will then either be identical or the same percentage ofavailable active power for that WTG. In both; the result is that the WTGwith the highest available active power will contribute the most to theWPP power output.

In a further embodiment, the available turbines are divided in twogroups where one group will comprise controllable turbines and the othergroup will comprise the non-controllable turbines. Only the controllableturbines will have equalized curtailment.

In case of high wind; the controllable turbines have set-points close totheir possible power. Thus, selected turbines can be moved from thecontrollable groups to the non-controllable groups and be assignednominal power. This may be achieved by having an additional clause formoving WTGs into the non-control subset or by setting the predeterminedvalue such that the desired number of turbines is moved to thenon-control subset. This is to increase the capability of the remainingcontrollable turbines by increasing the span between possible power andproduction reference. Furthermore this will avoid running turbinescurtailed and close to possible power.

This embodiment of the equalized curtailment does not require all WTGsrunning equalized curtailment as some turbines may be running withnominal set-point, but all curtailed turbines will be controlled towardsthe equalized curtailment.

In an embodiment, only a subset of the curtailed turbines in the WPPform part of the WTGs that are equalized.

The advantages of this embodiment is fast reaction and accuracy inactive power. As the curtailment is equalized, meaning that the WTGwhich are curtailed are curtailed by the same amount of active power.

The equalize capability strategy maximizes the wind farm performancecapability by attempting to ensure all controllable WTGs have the samepositive active power capability. The aim is that when demand increases,all the controllable WTGs are able to ramp to comply with the newdemand. Further, high producing turbines will decrease while lowproducing turbines with high wind will increase, such that over time allrunning WTGs will converge towards the same average positive capability.

FIG. 3 shows an example of the equalized curtailment for WTG(m) andWTG(n), where m and n are index numbers of the WTGs. The possible poweras a function of time is shown for both WTGs, where WTG(n) has a higherpossible production compared to WTG(m). Production is shown for time t1and time t2.

The WTG at positions m and n are compared, and capability is transferredbetween the two:CapToMove=min(EQ_Limit,abs(PosCapErrorN*k),abs(PosCapErrorM*k))ActivePowerSP_(WTG)[m]:=ActivePowerSP_(WTG)[m]+CapToMoveActivePowerSP_(WTG)[n]:=ActivePowerSP_(WTG)[n]−CapToMove

Where “CapToMove” is the capability, meaning the power or the power setpoint, which are moved from turbine n to m. The constant “k” is there toensure that only a portion of the power difference is equalized by eachdispatcher iteration.

In an embodiment the portion defined by the constant k is 20%, thusk=0.2 and the EQ_Limit=50 kw.

Therefore in the static state the turbines n and m will then have equalcurtailment after five dispatcher iteration. The real world is notstatic and therefore the constant “k” helps minimizing oscillations inthe algorithm. “k” may be understood as the length of the steps for eachiteration.

Further, “PosCapErrorM” is not an actual error, but denotes thedifference between the positive capabilities of the turbine M inrelation to the average positive capability of the WTGs. It is importantto emphasize that the capability here means the ability to deliverpower.

The constant k allows the convergence to slow proportionally as WTGsapproach the average capability and avoid oscillating around the targetvalue.

In practical terms the control subset is sorted two ways from highest tolowest based on the difference between the WTG's positive capability andthe average capability of all controllable WTGs.

This can be seen in FIG. 4, where the upper part of the Figure shows thesorting of the WTGs in respect of positive capability error startingfrom WTGm in the left side descending to WTGn in the right side. Thelower part of the Figure shows the sorting in the opposite direction,ascending from WTGn to WTGm. So in a situation where the curtailment isgoing to be equalized power can be moved from WTGn to WTGm or reverse.Thus, each column represents a WTG, and each WTG is represented in inboth the upper and the lower row. The arrow shows that a specific amountof production is moved from one turbine to another. In other words thatan amount represented by the column is subtracted from one turbinespower set point and added to another turbines power set point. Herebyequal curtailment is achieved.

The amount of capability moved may be further limited such that neitherWTG will be sent to the non-control subset. In the event that nocapability can be transferred, the limiting WTG will be ignored and thecapability will be transferred with the next WTG in the list.

In an embodiment where a WTG has stopped communicating, the WTG is movedto a separate non-communication subset to be ignored by whicheverdispatching strategy is being used, as the WTG does not provide acontrollable change in its power production.

It will be understood that the benefits and advantages described abovemay relate to one embodiment or may relate to several embodiments. Itwill further be understood that reference to ‘an’ item refer to one ormore of those items.

The invention claimed is:
 1. A method for operating a wind power plantwhich supplies power to a utility grid in accordance with an activepower reference, the wind power plant comprises a plurality of windturbine generators and a power plant controller arranged to send anactive power set point to a wind turbine controller of each of theplurality of wind turbine generators, the method comprising: operatingthe wind power plant with the active power reference lower than anominal active power of the wind power plant by curtailing at least someof the wind turbine generators of the plurality of wind turbinegenerators, determining a power capability of each of the curtailed windturbine generators, defining a first group comprising wind turbinegenerators of the curtailed wind turbine generators having a powercapability higher than a power capability limit, defining a second groupcomprising wind turbine generators of the curtailed wind turbinegenerators having a power capability lower than the power capabilitylimit, and decreasing the active power set point for the wind turbinegenerators in the first group and increasing the active power set pointfor the wind turbine generators in the second group such that a powerproduction of the wind turbine generators in the first group is reducedaccording to an average reduction from the power capability of the firstgroup and such that a power production of the wind turbine generators inthe second group is reduced according to the average reduction from thepower capability of the second group.
 2. The method according to claim1, wherein, determining a power capability of each of the curtailed windturbine generators comprises at least one of: calculating a positivepower capability, as the difference between possible power andproduction power; calculating a negative power capability, as thedifference between minimum power and production power.
 3. The methodaccording to claim 2, further comprising limiting the decrease or theincrease in the active power set point by an equalizing limit value in adispatcher iteration.
 4. The method according to claim 3, whereinlimiting the decrease or the increase in the active power set point isset as a minimum of the equalizing limit value and a portion of thepower capability.
 5. The method according to claim 1, further comprisingsetting a constant, prior the step of decreasing the active power setpoint for the wind turbines generators in the first group and increasingthe active power set point for the wind turbines generators in thesecond group, to ensure that only a portion of the power capability,defined by the constant times the power capability, is equalized in adispatcher iteration.
 6. The method according to claim 1, wherein thepower capability limit is a median or average power capability of thecurtailed wind turbine generators.
 7. The method according to claim 1,wherein the number of wind turbine generators in the first group and thesecond group are substantially equal.
 8. A wind power plant having aplurality of wind turbine generators and a power plant controllerarranged to communicate with the plurality of wind turbines generators,wherein: each wind turbine generator in the plurality of wind turbinegenerators being related to a wind turbine controller, the wind turbinecontroller being arranged to control an active power output in itsrelated wind turbine generator according to an active power set pointreceived from the wind power plant controller; the wind power plantsupplies power to a utility grid in accordance with an active powerreference; the active power reference lower than a nominal active powerof the wind power plant, such that at least some of the wind turbinegenerators of the plurality of wind turbine generators are curtailed;each of the curtailed wind turbine generators have a power capability;the curtailed wind turbine generators comprise: a first group comprisingwind turbine generators with a power capability higher than a powercapability limit; and a second group comprising wind turbine generatorshaving a power capability lower than the power capability limit, whereinthe wind turbine controller is arranged to decrease the active power setpoint for the wind turbines generators in the first group and increasethe active power set point for the wind turbines generators in thesecond group such that a power production of the wind turbine generatorsin the first group is reduced according to an average reduction from thepower capability of the first group and such that a power production ofthe wind turbine generators in the second group is reduced according tothe average reduction from the power capability of the second group.